The present invention relates generally to cooling of rotatable shafts, and more particularly, to cooling rotary shafts for use in orienting a substrate, e.g., a semiconductor wafer, in a path of an ion beam in an ion implantation system
In an ion implantation system, wafer holders are commonly employed in an evacuated chamber to position a wafer, e.g., a semiconductor substrate, in a path of an ion beam. Each wafer holder can be coupled to a rotating shaft to orient the wafer in a plurality of different orientations relative to the ion beam. The shaft typically extends from the vacuum chamber, through a vacuum feedthrough, to another region maintained at atmospheric pressure in which a drive mechanism for rotating the shaft is disposed. A conventional vacuum feedthrough can include bearings and a differential seal that provides a vacuum tight separation between the evacuated chamber and the region maintained at atmospheric pressure.
In use, the wafer holder positions a wafer in a path of the ion beam. Bombardment of the wafer by the ions will raise the wafer temperature, and consequently that of the wafer holder to elevated levels. The thermal contact between the rotary shaft and the wafer holder can in turn cause a rise in the temperature of the rotary shaft. In a conventional system, the rotary shaft can be cooled by circulating a cooling fluid, such as, water, through one or more internal channels in the shaft. This typically requires a flexible coupling of the shaft to a source of cooling water such that the shaft remains able to rotate while it is cooled. Such connections are, however, susceptible to failures that necessitate frequent maintenance, thereby decreasing productivity and increasing the cost of operating the system.
Thus, there is a need for improved cooling mechanisms for rotary shafts that extend between two regions having different ambient pressures and temperatures.
There is also a need for improved wafer holding assemblies having rotary shafts for use in ion implantation systems for orienting and positioning wafers.